Growth and reproduction of a short-lived cephalopod : mechanisms that facilitate population success in a highly variable environment

For short-lived species seasonal fluctuations in environmental conditions plays a major part in shaping population structure and dynamics. Cephalopods are known for their short life span, rapid growth, and early maturation. Small changes in environmental conditions have significant effects on life history characteristics, such as the timing and size at hatching, growth rates, longevity, and size and age at maturation. For cephalopods that live for less than a year annual environmental cues are not used to synchronise life events (e.g. timing of gametogenesis and spawning), instead individuals grow and reproduce throughout the majority of the year. As a result cephalopod populations are typically made up of multiple cohorts with differing life history characteristics, making it difficult to establish generalizations about the structure and dynamics of populations both within and among species. To date, both laboratory and field based studies have been useful in understanding some of the mechanisms responsible for the plastic characteristics that has allowed cephalopods to be so successful in response to the variable environmental conditions. The aim of this research was to use a small cephalopod species with a short life span, Euprymna tasmanica, to explore the relationships between growth and reproductive output from laboratory held populations alongside growth estimates from the field to explain some of the mechanisms that may be responsible for the seasonal variability found within populations of Euprymna tasmanica in northern Tasmania. In captivity E. tasmanica hatched at approximately 0.3g and the percent increase in body mass per day followed a biphasic growth pattern, starting with a fast exponential growth model followed by slower almost linear growth. Changes in temperature and ration had significant impact on the growth and reproductive characteristics, however in most the influence of temperature and ration were independent of each other. During the initial stage of growth higher water temperature was seen to significantly increase the rate of growth, while greater rations increased growth during the late phase of growth. An elevation of temperature of 5°C over the entire lifespan decreased the age and weight at first egg deposition by 16 days, and 0.89g respectively, and halved the average egg size. Females fed at a greater ration were 12 days younger at first egg deposition and produced eggs that were on average 25% larger; however, their size at first batch deposition (6.23g ± 0.19) was no different from those females fed a smaller ration. Only the number of eggs in the batch was affected by the interaction of temperature and ration, with individuals experiencing a combination of high temperature and ration producing average batch sizes of around 128 eggs which was approximately twice the size of the other treatments. Within the population of E. tasmanica sampled, it was apparent that the temperatures experienced had a significant effect on the growth, maturity and reproductive condition. Growth in terms of size-at-age was sex specific, with the temperature experienced having no effect on the growth of males. Females on the other hand grew larger when experiencing cold water, but this was likely to be a factor of living longer rather than growing faster. Immature individuals that had experienced cooling or cold water temperatures were also larger, suggesting that maturity occurs at larger sizes during winter. It seems that the direction of change in water temperature, rather than the temperature range alone influenced the condition of E. tasmanica, with individuals of both sexes experiencing warming water temperatures being in poorer reproductive and somatic condition compared to individuals who experienced cooling water temperatures. Histological analysis of the mantle muscle dynamics showed significant differences of muscle block width, fibre frequency and fibre diameters from different regions of the mantle, indicating that growth is not uniform throughout the whole mantle. Differences in muscle growth dynamics of squid was largely dependent on the water temperature individuals experienced. Biochemical indices were also used to examine differences in growth among squid that experienced different water temperatures. In this study RNA, protein and RNA:Protein ratio showed that growth is faster in smaller individuals and individuals that experienced warmer environments. Additionally, the level of both reproductive investment and reproductive status of individuals had no effect on any of the biochemical indices, suggesting that variability in growth rates is not a factor on reproductive investment and that the process of growth and reproduction may be independent of each other. The large amount of unexplained variation in these results however, suggest that even when taking the environmental influence and reproductive condition into account the growth in E. tasmanica remains highly variable and difficult to explain. By studying the influence of temperature and ration on the life history characteristics of individuals in the lab, alongside the assessment of a wild population, this study was successful in explaining how the structure and dynamics of a population of short-lived squid changes in response to short term environmental variability. While growth and reproduction progress together over much of an individual's life, it appeared that depending on the environment experienced individuals were able to switch between two main reproductive strategies, each using a different method to maximise population fitness. Individuals that experience warming and warm water were able to growth fast, due to an increase in hypertrophic growth, increasing fitness by reducing the time between generations. Individuals adopting this strategy appeared to have similar reproductive characteristics to that of a terminal spawning species. In contrast, individuals that experienced cooling and cold water grew slowly mainly through a hyperplastic dominated growth, reaching maturity later. Although this strategy increased the risk of being preyed upon before spawning, individuals were able to spawn multiple times over an extended lifespan, increasing the chance that some of their offspring will experience conditions favourable for survival. While it is likely that the reproductive strategy of individuals will fall somewhere in-between these two strategies, the ability of this short-lived squid to survive in a variable environment owes its success to its flexible reproductive strategies.